A SAW device produces a delayed echo when interrogated by an RF pulse (i.e., an interrogating signal or incident signal) from a radio frequency (RF) transmitter. The echo response or echo signal is produced passively, that is, similar to a light-reflecting mirror. However, like a distorting carnival mirror, the echo signal carries a signature of the reflecting device as well as a signature of the incident signal. The echo signal is re-transmitted from the SAW device to an RF receiver where it is analyzed to determine characteristics of the echo response and the material on which the SAW device mounted.
FIG. 1 depicts a prior art SAW device 410. An interrogating or illuminating wave comprises a radio frequency (RF) signal pulse 412 transmitted by an RF transceiver or interrogator 414. The transmitted signal travels through space to each SAW device within the field of view of the SAW interrogator 414. Only one SAW device 410 is illustrated in FIG. 1. The interrogating signal is received by an antenna 418 connected to an interdigital transducer (IDT) 420 disposed on a piezoelectric substrate 424. The IDT 420 launches an incident surface acoustic wave (SAW) 428 onto the piezoelectric substrate 424 in response to the received interrogation signal.
The SAW 428 propagates along the substrate 424 and is received at a reflector array 430 also disposed on the piezoelectric substrate 424. In applications where several SAW devices are within range of the RF transceiver each provides a reflected signal back to the RF transceiver. The reflector array 430 of each SAW device in a system of SAW devices (where the SAW device 410 is one SAW device within that system) comprises a unique pattern of metal electrodes 434 that impart a unique impulse response to the incident SAW 428. The impulse response of the reflector array 30 is imparted to the incident SAW 28 as it launches a reflected SAW 34 back to the IDT 420. The impulse response may include information about the spacing of the elements comprising the array 430, where a change in that spacing is caused, for example, by expansion or contraction of the substrate 424. The IDT 420 then converts the acoustic wave to an electrical signal that is radiated from the antenna 418 back to the RF transceiver 414 for extraction of the desired information in the reflected signal.
Surface acoustic wave (SAW) devices have been successfully used as temperature, mechanical, and rotational sensors, for example.